Look up DKIM public keys by selector, or auto-probe the 25 most common selectors used by Google, Microsoft 365, SendGrid, Mailgun, ProtonMail and others.
DomainKeys Identified Mail (DKIM) lets a receiving server cryptographically verify that an email was actually sent (and not modified) by the domain it claims. Each sender publishes one or more public keys at <selector>._domainkey.domain. There's no way to enumerate selectors via DNS, so you need to either know the selector (check your DNS zone) or probe common ones.